1. Field of the Invention
The present invention relates to an electron microscope designed to focus an electron beam onto a specimen by condenser lenses and an objective lens, to scan the beam across the specimen, to detect electrons transmitted through the specimen, and to image the resulting signal in synchronism with the scanning of the beam.
2. Description of Related Art
Where an electron microscope is operated in the STEM imaging mode, an electron beam produced from an electron gun and accelerated is sharply focused onto a specimen by an illumination lens system consisting of condenser lenses and an objective lens. A desired area on the specimen is scanned by the beam. Since the electron beam is directed at the specimen, some of the electrons are transmitted through the specimen. These electrons are detected. The resulting signal is supplied to a display device according to the scanning of the primary electron beam. Thus, a STEM image of the specimen is displayed.
In this electron microscope, the resolution of the final image depends on the degree of astigmatism in the electron beam hitting the specimen and on the degree of misalignment of the beam with respect to the optical axis. Therefore, a stigmator and an alignment device are attached to each lens. A STEM image is displayed in practice. The operator adjusts the stigmator and alignment device to optimize the state of the image while observing it.
However, it is quite difficult even for the skilled operator to correct astigmatism sufficiently or align the beam by adjusting the stigmator or alignment device while watching the STEM image. This astigmatic correction must be done during normal SEM imaging mode, as well as during STEM imaging mode.